In the last twenty years optical modulation techniques have become increasingly important in data communication systems. A variety of optical devices have been developed. In some applications, such as for obtaining return to zero (RZ) modulation formats, narrow-band modulators are of significant importance. One such device is a double cavity device (DCD), which consists of two coupled cavities. An RF field applied externally to the DCD determines the amount of optical power transferred to the output port of the device. Prior art describes only a double cavity device (DCD), but does not explain how to optimize the structure of a practical device.
A schematic illustration (top view) of a DCD is presented in FIG. 1. The device is implemented utilizing two coupled waveguides with highly reflecting mirrors at each end of the waveguides. A physical model and structure of coupled waveguide devices are presented in Griffel, G. (1993). “Very short intra-cavity directional coupler for high-speed communication.” Appl. Phys. Lett. 63(2): 135-7, Nir, D. PhD Thesis “Novel Integrated Optic devices based on irregular waveguide features” Tel-Aviv university 1996, and Nir, D. and S. Ruschin (1992). “Very short integrated optic Fabry-Perot Cavity-Coupler switch for DC and High bandwidth operation.” IEEE JQE 28(11): 2544-50, contents of which are hereby incorporated by reference. The DCD structure is described in international patent application number PCT/IL01/00196, the contents of which are hereby incorporated by reference. In the configuration shown in FIG. 1, two mirrors, 101 and 102, confine a Fabry-Perot cavity 103. The coupled waveguide in the cavity 103 support two or more normal modes (i.e., the zero order symmetric and anti-symmetric modes, the first order symmetric and anti-symmetric modes etc. . . . ), each having a different resonance frequency. A continuous wave (CW), or slowly varying light source, is injected to the device via the input waveguide 104. The light enters the cavity 103 through a semi-reflective input/output (IO) mirror 102. The injected light is distributed between the various normal modes according to the amount of spatial overlapping between each of the normal modes and the injected field. When the optical length of the cavity is properly phase-tuned, the injected light of a specific normal-mode is amplified to a level depending on the structure parameters. The phase tuning determines which of the normal modes is amplified. All of the normal modes are coupled to the output waveguide 105 via the IO mirror 102. The phase difference between the modes, as well as the power of the various normal modes in the cavity, determines the amount of power that couples to the output waveguide 105. Thus, for example, if the symmetric and anti-symmetric modes are added in-phase with identical power carried by each mode, a vanishingly small power couples to the output waveguide, whereas if they are added with a 180° phase difference the output power at the output waveguide 105 is maximal.
To control the transmission state of the device, an electrical radio frequency (RF) voltage signal is applied to an electrode set properly positioned about the waveguides. The electrical field produced by the electrodes modifies the optical refractive index of the waveguides at the interaction zone via the electro-optic effect, affecting both the coupling and the relative phase between the normal modes in the cavity, and therefore modulating the output transmission.
Optimal performance of the device (i.e. an infinitely large extinction-ratio with low attenuation and with very low RF drive voltage) is obtained if the applied RF narrow-band field resonantly amplifies the coupling between the modes. No method currently exists for optimizing DCD performance. It is not currently possible to provide the conditions needed to bring the DCD to the optimal performance level. Additionally, prior art considers only a dual-cavity device, but not devices utilizing more than two cavities or devices having optical gain within the modulator.